In accessing a desired or target track on a disk that stores data or other information, servo control is utilized in order to properly perform the track seek operation. In a first known optical disk drive system, continuous seek-related information is provided on the disk by a continuous pregroove, which generates a radial push-pull signal from which track counts can be derived. Based on the number of track counts over a measured time period, the actual velocity of a seek actuator can be calculated, the seek actuator being used in moving the light beam across disk tracks. The magnitude of the actual velocity is compared with a predetermined and desired velocity whereby a velocity error signal can be found. The velocity error signal is outputted by a seek servo loop which is used to control the velocity of the seek actuator so that the light beam is properly positioned relative to the desired track. This known method has a number of disadvantages. Data pits formed in the tracks interfere with proper counting of the tracks by means of the continuous pregroove. The magnitude of the seek velocity is thereby reduced by such interference. In this method, the performance of the seek operation is dependent upon the data modulation format stored in the tracks of the disk, which limits the user to a certain data format. Also, a relatively significant amount of analog signal circuitry is required to remove the data signal from the track count signal, e.g., filters, sample and hold circuits, automatic gain control circuits, and comparators.
In a second known method for track seeking in an optical disk system, track seeking is accomplished using two different techniques, depending upon the length of the seek or the distance the desired track is from the current position of the light beam used in reading or writing. In this system, for relatively short seeks, conventional methods for track counting are utilized, such as relying on servo information stored or provided on the disk surface. For relatively long seeks, a sensor external to the disk is utilized in arriving at the number of tracks that were crossed in any given time period. The external sensor controls a coarse actuator used in positioning the light beam. As an example of an external sensor, a Moire type optical ruler is used until it is determined that the desired track being sought is within a predetermined number of tracks relative to the current position of the light beam. Such hardware and method results in additional costs for the external sensor and accompanying electronics. This particular two-step technique is also a relatively cumbersome implementation of a seek operation.
Also previously devised is the use of an alternating pit position in servo bytes stored on a disk track. In Netherlands App. No. 8600934, filed Apr. 14, 1986, corresponding to European Pat. Appl. No. 0241978, and entitled "Optical Record Carrier and Apparatus For Reading the Record Carrier," the use of an alternating pit is disclosed. A pit is provided in a selected one of a number of servo byte positions for a predetermined number of tracks and then the pit position is changed for the next predetermined number of tracks. This altenating of the pit position is continued. The alternating pit is used in providing track addressing information. The use of the alternating pit is employed in the present invention. However, unlike the previous application, the use of the alternating pit is in connection with velocity feedback in order to provide an accurate and controlled seek operation.
The known prior art does not address problems and solutions thereto that were encountered in connection with devising and/or implementing the controlled seek operation of the present invention, which relies on servo information that is inermittently stored along each disk track. A major objective of a track seek operation is to minimize the time taken to reach or capture a desired or destination track, as well as to avoid undershooting or overshooting of the desired track. In achieving this objective in the system of the present invention which relies on sampled information stored or provided on the disk surface, two competing considerations are taken into account or balanced. First, the actuator velocity must be sufficiently great at the high end of the velocity spectrum of the seek actuator to minimize the access time to the desired track while avoiding unwanted "folding or aliasing" effects. Second, underdamping of the servo loop should be avoided or minimized. With regard to the first consideration, folding occurs when the actuator velocity is equal to or greater than a certain velocity in a sampled seek system. For example, in the case in which servo information repeats every 20 microseconds for a given rotational speed of the disk, then velocities which are above 50 kHz (1/20 microseconds) cause folding. That is, because the servo information is intermittently located along the disk tracks, above a certain velocity, tracks being crossed will not be counted. The velocity of the actuator is so great relative to the rotational speed of the disk that servo information for one particular track may not be accessed during the passage of the light beam across the one particular track. To overcome this problem, a track counting method is employed by the present invention whereby a significantly much greater velocity of the actuator must be exceeded before folding occurs. This selected counting method is intended to allow the actuator to move as fast as is desired in the particular application.
However, the use of such a counting method also results in the establishment of a seek actuator minimum velocity below which the servo loop system does not properly function, i.e., reliance on such a counting method below the minimum velocity would result in an underdamped servo loop. To overcome this problem relating to loop stability, another counting method was arrived at for deployment when the actuator velocity was below this minimum velocity. It was determined that this other counting method has associated with it a maximum actuator velocity that is less than the aforesaid minimum actuator velocity. Because of this difference, there is a "velocity gap region" between the maxiumu velocity of the one counting method and the minimum velocity of the other counting method. The present invention identifies various methods that can be utilized when the actual actuator velocity corresponds to one of the velocities identified as being in the velocity gap region. A discussion of such solutions and the preferred methods of counting tracks crossed, depending upon the current seek actuator velocity, is provided in the following descriptions of the embodiments of the present invention.